WO2019219930A1

WO2019219930A1 - Drainage of cellulose nanomaterials

Info

Publication number: WO2019219930A1
Application number: PCT/EP2019/062840
Authority: WO
Inventors: Jatin SETHI; Kristiina Oksman; Mirja Illikainen
Original assignee: University Of Oulu
Priority date: 2018-05-17
Filing date: 2019-05-17
Publication date: 2019-11-21

Abstract

The present invention relates to a method for preparation of a modified cellulose material, wherein the method comprises sonication of a mixture comprising a suspension of cellulose and an organic acid. It also relates to the cellulose material obtained by the method, a paper sheet, a cellulose nanopaper, as well as the use of the method for reducing the drainage time in processing of cellulose into finished products.

Description

DRAINAGE OF CELLULOSE NANOMATERIALS

FIELD OF THE INVENTION

The present invention relates to a method for preparation of a modified cellulose, wherein the method comprises sonication of a mixture comprising a suspension of cellulose material and an organic acid.

TECHNICAL BACKGROUND

Cellulose is well known for its contribution to human advancement. It has been a source for paper, textile and films for the past 150 years. Lately, it has come to light for its extraordinary properties in nanoscale. This has opened the door for use of naturally occurring materials into advanced functional products such as composites and flexible electronics. Due to such potential applications, cellulose nanomaterials are most sought-after materials today. They support a bio-based economy, and their raw material, especially wood, is the most abundant material in the world. They are also known to be useful in paper processing as strength enhancer and rheology modifier. One of the most discussed nanomaterials is cellulose nanopaper. It is an ultra-strong paper, prepared by draining water from cellulose nanofiber (CNF) suspension, followed by drying. The reason for its popularity is its extra ordinary strength, light weight structure and being derived from a renewable resource. Despite of its numerous advantages it suffers from the major drawback that it loses its properties in presence of large amounts of water, such as in high humidity, the reason being the hydrophilic nature of cellulose. The hydrophilicity of CNFs also causes the process of draining water from CNF suspensions to take an extraordinary long time. The water retention of cellulose nanoscale is exponential when compared to paper pulp, due to drastic increase in surface area. Drainage or dewatering is the most energy consuming industrial scale process. Any extension of the time for performing this step cause huge rise in production costs. This prevents the use of CNFs in finished products such as papers, liners boards, or nanopapers. This problem has been previously solved by using external chemicals. One way is to use the charged polymers such as poly(diallyldimethylammonium chloride) or PDADMAC, which was shown by Ahola et al., BioResources 3 (2008) 1315-1328. Rantanen et al. Cellulose 22 (2015) 4003-4015, used high concentration of precipitate calcium carbonate (PCC), to decrease the drainage time in nanocellulose and pulp mixture. For nanopapers, Varanasi and Batchelor, Cellulose 20 (2013) 211-215, used highly concentrated microfibrillated cellulose MFC under high pressures and filter papers with bigger pore size. Sim et al. discussed the effect of salt, NaCI, ( Cellulose 22 (2015) 3689-3700), on the drainage time of water from CNF suspension (1.5%).

SUMMARY OF THE INVENTION

One object of the present invention is to provide a method for preparation of a modified cellulose, wherein the method comprises sonication of a mixture comprising a suspension of cellulose material and an organic acid. The method implies an improved water drainage from a suspension of cellulose. The method presented herein is environmentally friendly and can be adapted industrially by utilizing cellulose fibres of different size scales in a traditional papermaking process or when processing it for other potential applications.

The method according to the present invention comprises sonicating a mixture of an organic acid and a suspension of cellulose material until the sonication energy is between 5-1000 J/ml to obtain a modified cellulose.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 presents the drainage time versus the sonication energy imparted for a non- modified CNF suspension (square■) and a CNF modified with lactic acid at a 1:1 ratio (star ★), and the drainage time of a non-modified CNF suspension and a CNF suspension in the presence of lactic acid at a 1:1 ratio that has not been subjected to sonication (circle · and triangle T respectively).

Figure 2 presents the water retention value (WRV) of cellulose nanofibers (CNF) after modification by lactic acid (LA) with different LA/CNF ratios. Figure 3 presents the drainage time of different CNF suspensions.

Figure 4 shows tensile testing curves for reference CNF nanopaper (solid line— ), CNF(10)LA nanopaper (dash-dot line -· · ) and CNF(0.1M)NaCI nanopaper (dotted line · · · )·

Figure 5 illustrates the water absorption of room dried non-modified CNF and CNF modified with NaCI and lactic acid.

Figure 6 shows FESEM micrographs of surface (upper micrographs) and cross-section (lower micrographs) of reference CNF nanopaper ((a) and (d), respectively), CNF(0.1M)NaCI nanopaper ((b) and (e)), and CNF(10)LA nanopapers ((c) and (f)).

Figure 7 presents the warpage in the reference CNF nanopaper (a, d), CNF(0.1M)NaCI nanopaper (b, e), and CNF(10)LA nanopapers (c, f).

Figure 8 presents the drainage time for preparation of 120-140 g/m² paperboards without CNF, and with CNF (10 wt. %), with and without modification.

Figure 9 presents the elastic modulus of paperboards without CNF, and with CNF (10 wt. %), with and without modification.

DETAILED DESCRIPTION OF THE INVENTION

A first aspect of the present invention relates to a method for preparation of modified cellulose, wherein a mixture of an organic acid and cellulose material in a suspension medium is sonicated until the sonication energy is between 5-1000 J/ml. It should be noted that features and/or advantages described in the context of one of the aspects of the present invention may also apply mutatis mutandis to all the other aspects of the invention.

The chemical reaction occurring when the mixture of an organic acid and a cellulose material in a suspension medium is sonicated according to the present invention is shown in the below scheme, where the organic acid is represented by lactic acid and the cellulose material by cellulose nanofibers.

OH OH OH O

CNFs Lactic acid LA-g-CNFs

The carboxylic group of the organic acid reacts with hydroxyl group of cellulose in presence of sonication. Using sonication according to the present invention has the advantage that a catalyst is not required for the chemical reaction between the organic acid and the cellulose material. Thus, the method according to the present invention does not require the presence of additional chemicals, for example catalysts, such as Lewis acids. Consequently, the modified cellulose may be produced without the presence of a substance acting as a catalyst for the reaction between the cellulose and the organic acid. This enables the production of a pure modified cellulose with a minimum of components and without encountering potential problems with removal of the catalyst from the final product. Further, the method only requires moderate amounts of the organic acids used for modification of the cellulose. The ratio of organic acid to cellulose in the mixture of an organic acid and cellulose material in a suspension medium may be from 0.01:1 to 20:1, from 0.01:1 to 10:1, from 0.05:1 to 10:1, from

0.1:1 to 10:1, or from 0.1:1 to 1:1, as calculated by dry weight of cellulose. Typically, the quantity of organic acid may be very small, such as less than 0.5 wt% in water. The organic acid may have a pKa of 1.0-6.0, preferably 3.0-5.0. Additionally, the method may use organic acids that are biobased. Suitable organic acids for use in the present method are selected from any acid of the formula CH₃-(CH₂)n-COOH, CH₃-(CH₂)_n-C(OH)-COOH, HOOC-(CH₂)_n-C(OH)-COOH, HOOC-COOH,

HOOC-(CH₂)n-C(OH)(COOH)-(CH₂)n-COOH, HOOC-(CH₂)n-[C(OH)]_n-(CH₂)_n-COOH, and Ph-COOH or a combination thereof, wherein each n independently is an integer from 0-6. Acids comprising more than one reactive group, such as alpha-hydroxy acids, for example glycolic acid, lactic acid, malic acid, citric acid and tartaric acid, may polymerize through self-condensation to form long polymer chains grafted on to the cellulose nanomaterial. Exemplary organic acids for use in the present method are selected from any one of acetic acid, propionic acid, butyric acid, lactic acid, malic acid, citric acid, tartaric acid and benzoic acid. Preferably the organic acid is lactic acid.

The term cellulose material is used herein for cellulose fibres, cellulose nanomaterial and cellulose filaments. The cellulose fibres may be natural fibres or manufactured cellulose fibres. Natural cellulose fibres are typically composed of cellulose microfibrils in a matrix of hemicellulose and lignin. Manufactured cellulose fibres may be obtained by extrusion from a cellulose suspension. The method according to the present invention particularly benefits the field of cellulose fibre networks and cellulose nanomaterials as reinforcement for paper sheets. The cellulose material in the suspension that is mixed with an organic acid and sonicated, according to the method disclosed herein, may be a cellulose nanomaterial. The term cellulose nanomaterial is defined in ISO standard ISO/TS 20477-2017. The term cellulose nanomaterial is thus used herein for nano-structured cellulose with any external dimension in the nanoscale, i.e. from 1 nm to 100 nm, or a material having internal structure or surface structure in the nanoscale, and includes nanofibrillar cellulose (NFC), microfibrillated cellulose (MFC), cellulose nanofibers (CNF), and cellulose microfibrils (CMF), which are liberated from cellulose plant material or occurs in bacterial cellulose; or cellulose filaments in nanoscale that are extruded or electrospun from cellulose suspensions. Preferably, the term cellulose material as used herein excludes cellulose nanocrystals. The present invention also relates to the preparation of modified cellulose fibres and filaments with any external dimension in the micro-scale. Preferably, the method according to the present invention is for preparation of a modified cellulose nanomaterial. The terms "CNF" and "MFC" are used interchangeably herein for cellulose nanofibers, also called microfibrillated cellulose, that are liberated from wood pulp or from other sources, for example selected from the group consisting of plants, tunicate, and bacteria by means of mechanical disintegration. The cellulose material used in the present method is preferably derived from plants, including wood. Plant- derived cellulose nanofibers are often preceded by a chemical pre-treatment, such as by oxidation with 2,2,6,6-tetramethylpiperidine-l-oxyl (TEMPO) giving TEMPO- oxidized CNF, or by carboxymethylation giving carboxymethylated CNF; or by enzyme- treatment, such as by endoglucanases, giving enzymatic CNF. CNF and MFC typically have a smallest dimension in the range 2-100 nm, while the length can be several micrometres, such as up to 10 pm, and therefore the aspect ratio of CNF (ratio of length to diameter) is very large. The term "bacterial cellulose" is used herein for cellulose produced by bacteria, such as from Gram-negative bacteria species such as Acetobacter, Azotobacter, Rhizobium, Pseudomonas, Salmonella, Alcaligenes, and Agrobacterium, and Gram-positive bacteria species such as Sarcina ventriculi. The bacterial cellulose fibres have a large fibre surface which causes strong interactions with compounds with functional groups which readily form hydrogen-bonds, such as water. The cellulose nanomaterial used in the method preferably comprises microfibrillated cellulose (MFC), cellulose nanofibers (CNF), cellulose microfibrils (CMF), nanofibrillar cellulose (NFC) and bacterial nanocellulose. The suspension medium for the cellulose material, and preferably when the cellulose material is a cellulose nanomaterial, may be selected from an organic solvent, such as DMAc, DMF, dioxane, DMSO, acetonitrile, isopropanol, or methanol; an aqueous solvent, such as ethanol in water; an aqueous solution, such as a NMMO or a salt, e.g. NaCI, dissolved in water; or water; or different combinations thereof. The concentration of cellulose material in a suspension used in the method according to the present invention is from 0.1 to 20 wt%, from 0.1 to 15 wt%, from 0.1 to 10 wt%, or from 0.1 to 5 wt%, before the organic acid is added to the suspension. When the cellulose material is a cellulose nanomaterial, the concentration thereof in the suspension is preferably from 0.1 to 5 wt%, or from 0.1 to 2 wt%, before the organic acid is added to the suspension.

Sonication is known for making miniscule vacuum cavities in the liquid medium, which on collapsing gives a temperature of 5000 K and pressures of 1000 ATM and such extreme conditions may produce chemical reactions. The sonication used in the method according to the present invention may be performed until the sonication energy is between 5-1000 J/ml, or 5-800 J/ml, or 5-600 J/ml, or 5-400 J/ml, or 100- 1000 J/ml, or 100-800 J/ml, or 100-600 J/ml, or 100-400 J/ml,. The amount of sonication can be quantified according to Equation 1

(Equation 1)

wherein, E is the sonication energy in J/ml, P is the power of the sonicator in watts, t is the time of sonication in seconds, and V is the volume of sonicated liquid or suspension in millilitres. Although some heat may be evolved when sonicating the mixture, it is not required to heat the mixture of cellulose material and organic acid further to obtain the modified cellulose. The sonicated mixture may be kept at a temperature of at most 90 °C, such as at a temperature from 1 to 90 °C, from 1 to 75 °C, or from 1 to 60 °C.

The method according to the present invention may further comprise a step of removing the suspension medium from the modified cellulose obtained by sonication, for example by draining the suspension medium. Drainage may be performed by vacuum filtration, pressure filtration or evaporation or a combination thereof. An advantage with the method according to the present invention is that moderate vacuum levels are sufficient when vacuum filtration is used. An additional advantage is that the drainage time can be decreased to 75% or lower, such as to 50%, or even down to 25%, of the original drainage time for cellulose fibres of different size scales (such as cellulose fibres; cellulose nanofibers (CNF), microfibrillated cellulose (MFC), cellulose microfibrils (CMF), and nanofibrillar cellulose (NFC) that are liberated from cellulose plant material or occurs in bacterial cellulose; or cellulose fibres or filaments in micro- and nanoscale that are extruded or electrospun from cellulose suspensions; or mixtures thereof). Further, with the present method the drainage time may also be considerably reduced compared with the drainage time for draining of the suspension medium from sonicated suspensions of non-modified CNF. In fact, sonication of a suspension comprising non-modified CNF may even increase the drainage time compared with non-sonicated suspensions of non-modified CNF, which makes it even more unexpected that sonication according to the present invention may enable a reduced drainage time. Moreover, the drainage time is also improved for cellulose pulp containing cellulose fibres, or for mixtures comprising cellulose pulp and cellulose nanomaterial, that are modified according to the method of the present invention. Drainage may be performed immediately after sonication, or the modified cellulose material may be retained for a period after sonication, preferably in a temperature not higher than 90 °C, such as at a temperature from 1 to 90 °C, from 1 to 75 °C, or from 1 to 60 °C, before the suspension medium is removed. Such period need not be longer than a couple of seconds. Such period may be quite long, such as from 0.5 seconds to several hours, for example up to 24 hours, but it may also be short, such as from 0.5 seconds to 1 minute, or 0.5 seconds to 1 hour. The method may further comprise a step of drying the modified cellulose nanomaterial, preferably to achieve a modified cellulose nanomaterial having a dry content of at least 95 wt%, or at least 98 wt%. The drying may be made after drainage of the suspension medium. Drying may be performed by compression moulding or casting the modified cellulose nanomaterial to obtain a film or nanopaper, or by subjecting it to an elevated temperature, such as above 100 °C, to evaporate the remaining suspension medium, or a combination thereof.

The method according to the present invention provides a cellulose nanomaterial modified with an organic acid. Thus, the present invention also relates to a modified cellulose nanomaterial prepared with the method according to the present invention. The modified cellulose nanomaterial obtained with the method according to the present invention may have a water retention value, WRV, of from 20-40. The WRV is defined herein by the ratio of the mass of water retained by a wet modified cellulose material, after centrifugation (3000 g, 10 minutes, 23 °C) and removal of the supernatant, to the oven-dry mass (100 °C for 24 hours) of the same modified cellulose material. The oven-dry mass obtained by drying the centrifuged modified cellulose material, or a cellulose nanomaterial, at 100 °C for 24 hours is herein termed mass of dry cellulose material, or mass of dry cellulose nanomaterial, respectively. WRV indicates the amount of water held by outside layers of fibres, and is determined according the following formula:

(Equation 2)

where W_w is the weight of the wet sample after centrifuging, and 1/14_/ is the weight of the dried sample. A modified cellulose material prepared according to the present invention will also achieve an improved elastic modulus compared to the corresponding non-modified cellulose material. In a further aspect the present invention also relates to a cellulose nanopaper prepared from the modified cellulose obtained by the method according to present invention. An advantage with nanopapers prepared from the modified cellulose according to the present invention, is that they have a reduced water absorption compared to the nanopaper prepared from the corresponding non-modified cellulose. In yet a further aspect, the present invention also relates to the use of the method according to the present invention for reducing the time required for water drainage from suspensions based on a cellulose material, such as in a process for preparing a paper sheet, board, liner, or nanopaper.

EXPERIMENTAL EXAMPLE 1

Materials

A cellulose nanofibers (CNF) suspension with a dry content of 1.6 wt% was prepared from bleached soft wood sulphite fibres (Stora Enso, Oulu, Finland) with hemicellulose and lignin contents of 4.2 wt% and 0.3 wt% using a Masuko^® grinder (contact mode). The positions of the disks, according to selectable positions in the Masuko^® grinder, and the number of passes through the grinder are given in Table 1. Table 1

CNF modification

For modification with organic acid, the CNF suspension in water was mixed with lactic acid (LA) according to the formulation in Table 2. The coding of samples was done according to ratio of CNF and LA in the suspension. For example, a sample with a dry weight of LA 10 times the dry weight of CNF is coded as CNF(10)LA and according to the formulation in Table 2 it will contain 0.35 wt% CNF and 3.5 wt% LA. In a typical modification, 200 gr of CNF(10)LA was prepared by taking 43.75 g of the water based 1.6 wt% CNF suspension (i.e. 0.7 g of CNF material) and mixing it with 7 g of lactic acid and 149.25 g water. The cellulose nanofibers (CNF), water and LA were mixed with a high speed stirrer (Ultraturrax) at 1500 rpm for 5 minutes and thereafter sonicated with a 22 mm titanium tip of probe type sonicator (Hielscher UP 400s) in a pulse mode with a 50% duty cycle setting (i.e. on for 0.5 sec, then off for 0.5 sec, repeated). Sonication energy was used to quantify the amount of sonication applied to the mixtures (Equation 1). Multiple samples were prepared from each formulation, and the different samples were subjected to different sonication energies, respectively, which are specified in the results for each performed test described herein. The maximum sonication energy imparted was 600 J/ml (10 minutes of sonication in a pulse mode with a 50% duty cycle setting). (Equation 1)

where E is the sonication energy in J/ml, P is the power of sonicator in watts, t is the time of sonication in seconds, and V is the volume of liquid in millilitres. Table 2

For comparison, CNF(0.1M)NaCI was prepared by adding NaCI to a suspension of 0.35 wt% CNF in water until the concentration of NaCI in the suspension reached 0.1 M, followed by mixing for 5 minutes with a high-speed stirrer (Ultraturrax) at 1500 rpm. As reference, three different CNF suspensions were prepared, all of them with a 0.35 wt% CNF and subjected to 5 minutes high speed mixing (Ultraturrax) at 1500 rpm: a non-modified CNF suspension in water; a non-modified CNF suspension further subjected to sonication (600 J/ml); and a suspension containing a mixture of CNF and LA with a ratio 1:1 prepared by only mixing both components with a high speed stirrer (Ultraturrax) for 5 minutes at 1500rpm (no sonication applied).

CNF-pulp modification·.

For modification with organic acid, 300 grams of 0.2 wt% pulp fibres aqueous suspension was mixed with 3.5 g of 1.6 wt% CNF water suspension and 0.1 gr lactic acid. The cellulose fibres, water and LA were mixed with a high speed stirrer (Ultraturrax) at 1500 rpm for 5 minutes and thereafter sonicated with a 22 mm titanium tip of probe type sonicator (Hielscher UP 400s) in a pulse mode with a 50% duty cycle setting till energy of sonication 100 J/ml (LA-CNF-pulp sample). For comparison, a non-modified suspension of pulp fibres and CNF was also prepared by mixing 300 g of 0.2 wt% pulp fibres suspension and 3.5 g of 1.6 wt% CNF water suspension for 5 minutes at 1500 rpm with an Ultraturrax (CNF-pulp sample). And as reference, a 300 g of 0.2 wt% pulp fibres suspension was also prepared without the presence of CNF by mixing it for 5 minutes at 1500 rpm with an Ultraturrax (pulp sample). Nanopaper preparation

Nanopapers prepared from suspensions of from CNF(0.1 M)NaCI, CNF(10)LA (modified with a sonication energy of 600 J/ml), and non-modified CNF as reference, were prepared by diluting the suspensions to a concentration of 0.2 wt% of cellulose material and draining the water under vacuum (70 ± 10 kPa) through a 0.65 pm Durapore PVDF membrane filter (Fisher Scientific, Pittsburgh, USA) mounted on an Erlenmeyer flask fitted with a vacuum pipe connected to a vacuum pump. After drainage, wet CNF (non-modified and modified) were kept under steel mesh and paper boards and were compressed and heated at the temperature of 100 °C and at the pressure of 10 MPa for 30 minutes to remove water. The LA modified nanopaper were further processed for 30 minutes at 150°C (under 10 MPa pressure), to increase the yield of esterification.

Pulp paper sheet preparation

120-140 g/m² pulp paper sheets (reference without CNF, CNF reinforced, and modified

CNF reinforced) were prepared from the pulp-CNF suspensions (pulp, CNF-pulp and LA- CNF-pulp respectively). The paper sheets were prepared by filtrating the pulp-based suspensions in similar conditions as in the nanopaper preparation and drying the wet pulp based cake in a Rapid-Kothen sheet former (Haage-Sheet Former, Estanit GmbH) under the vacuum of 1 bar and temperature of 95 °C for 10 minutes.

Characterization

The drainage time was characterized by measuring drainage time while the CNF suspension drains through a 0.65 pm Durapore^® PVDF membrane filter under vacuum (70 ± 10 kPa). The end point of drainage was assumed to be the instant when the difference between two consecutive drops was more than 30 seconds. 120-140 g/m² pulp paper sheets with 10 wt% non-modified and LA modified CNFs, respectively, were also drained under similar conditions. The pulp concentration was 0.2 wt.%. For settling studies, 250 ml CNF suspensions (modified and reference) at 0.05 wt% was allowed to settle overnight in a measuring cylinder.

For water retention value (WRV), CNF suspensions (reference and modified) were centrifuged at 6500 rpm for 10 minutes at room temperature (3000g force). The separated water was discarded and the amount of water retained by the CNF fibres was determined by the formula mentioned in Equation 2. The modified cellulose suspensions were prepared according to the formulations in Table 2. All the suspensions were supplied sonication energy of 250 J/ml.

(Equation 2)

Where W_w is the weight of the wet sample after centrifuging, and 1/14_/ is the weight of the dried sample.

Water absorption was determined by cutting dry nanopapers into pieces and weighing each nanopaper piece. The nanopapers were dipped in water overnight. The following day the wet nanopaper pieces were taken out from the water. Excess water that had not been absorbed by the nanopaper was removed from the nanopaper surface by gently wiping it with tissue paper. After removal of the excess water the soaked nanopapers were weighed and the water absorption was expressed in percentage according to Equation 3:

100 (Equation 3)

where W_s is the weight of the soaked nanopaper and 1/14_/ is the weight of the dry nanopaper.

Tensile testing was conducted for CNF(10)LA, CNF(0.1M)NaCI and reference nanopaper. Samples were conditioned in controlled conditions of room temperature and 50% humidity for 48 hours prior to testing. The force was applied from the load cell of 1 KN with the crosshead speed of 5 mm/min. The gauge length was kept at 20 mm. Standard procedures were followed to determine the elastic modulus and yield strength of the samples. An average of minimum 5 specimens per sample are reported. Paper sheets prepared from pulp, CNF-pulp and LA-CNF-pulp were also characterized according to the procedure for the nanopaper tensile testing.

Warpage was visually determined after keeping the nanopapers in controlled conditions (23°C and relative humidity (RH) of 50 %) overnight after preparation.

Results

Drainage time

Figure 1 shows the change in drainage time with increase in sonication energy for CNF(1)LA sample (star★). The drainage time decreased with increasing sonication energy. The reference CNF suspension without LA modification and without sonication treatment (circle ·) took around 45 minutes for the dewatering. On the other hand, after LA modification the drainage time was brought down to between 23 and 10 minutes depending on the used sonication energy. Sonication treatment in the absence of LA lead to increase of drainage time to 50 minutes (square ■), while addition of LA to nanocellulose without sonication (triangle Y ) gave a drainage time of near 30 minutes. The quantitative results of drainage time for 200 gr of CNF suspensions with various amount of LA is presented in Table 3.

Table 3

It can be observed that drainage time is dependent on sonication energy and is rather independent of amount of LA in the solution. Comparison of drainage time between LA modification and NaCI modification is presented in Figure 3. CNF(10)LA (modified with a sonication energy of 600 J/ml), took 10 minutes to drain water while CNF(0.1M)NaCI took 23 minutes to drain, and the reference had a drainage time of 45 minutes.

Settling study

After letting the reference and CNF(1)LA settle overnight, it was observed that CNF(1)LA had a higher volume of CNF at the bottom of measuring cylinder. It settled into 90 ml and 100 ml mark for 5 second and 60 second sonication respectively, while the reference had a lower volume (70 ml). From the drainage results, it can be inferred that modified CNFs assist the water drainage.

Water retention

Figure 2 presents the water retention values (WRV), which indicates the ability of CNFs to retain water, of non-modified cellulose nanofibers (CNF) and CNF after modification by lactic acid (LA) with different LA/CNF ratios and a sonication energy of 250 J/ml. It can be noticed that after LA modification, CNFs are losing water binding capacity .

Water absorption

Figure 5 presents the amount of water absorbed by reference, CNF(0.1 M)NaCI and CNF(10)LA. The modified nanopaper absorbs 80 % less water than reference and 100% less water than CNF(0.1M)NaCI, indicating that presence of hydrophobic moieties such as LA on the surface is rendering the nanopaper hydrophobic.

Tensile testing

Stress strain curves from stress-strain analysis of reference, CNF(10)LA and CNF(0.1M)NaCI nanopapers are presented in Figure 4 and quantitative results with statistical data are presented in Table 4. Table 4

[ *) Means that are marked by different superscript letters within the same column are significantly different from each other at 5% level based on the analysis of variance (ANOVA), e.g. measured values having the same superscript are thus not significantly different from each other.]

The elastic modulus of LA modified nanopaper (CNF(IO)LA) (modified with a sonication energy of 600 J/ml) is 41% higher than reference. On the other hand, yield strength is 60% higher than reference. Conversely, NaCI modified nanopaper is losing its modulus by 11%, yield strength is decreased by 31%. The morphology of different nanopapers prepared from reference, CNF(0.1 M) NaCI and CNF(10)LA was recorded by SEM and is presented in Figure 6. No apparent difference in surface characteristics is observed from the top-view. Cross-section imaging reveals that CNF(10)LA has different morphology, with CNFs glued within the layers. LA has a hydroxyl group and carboxylic groups that can polymerize through self-condensation to form long poly-LA chains. The presence polymerized LA may act as a glue between fibrils, which binds the fibres with each other improving the elastic modulus and yield strength.

Dimensional stability

Dimensional stability is an important parameter for commercial materials and moisture absorption is known to decrease the dimensional stability of plant-based materials (Cunha, A., et al., Cellulose 2014, 21 (4), 2773-2787). It can also be observed in Figure 7, which are the photographic images of nanopapers stored in 50% humidity and 23 °C overnight. Reference nanopaper and NaCI nanopaper are visibly warped. On the other hand, LA modified is clearly resisting the warpage. Drainage time of CNF-pulp suspensions and tensile testing of pulp paper sheets

Figure 8 presents the drainage time for pulp, and CNF-pulp and LA-CNF-pulp. The CNF- pulp suspension (without modification) took 23 minutes. The reference pulp without CNF took 3 minutes. On the other hand, LA modified CNF-pulp suspension 2 minutes. The elastic modulus of the paper samples prepared from these pulp-based suspensions is presented in Figure 9 and quantitative results from tensile testing is presented in Table 5.

Table 5

The tensile strength was improved for both the CNF and the LA-CNF reinforced papers. EXAMPLE 2

Materials

Cellulose fibre modification with lactic acid

30 gr (dry weight) of softwood pulp (Stora Enso Kraft softwood, unrefined) was disintegrated according to ISO-5263-1, resulting in a 2 L cellulose fibre suspension with a dry content of 1.5 wt%. Two 400 ml aliquots, containing 6 gr of dry weight pulp each, were taken from the 1.5 wt% disintegrated suspension and 1 gr of lactic acid (LA) was added to each sample. The samples had a dry weight of LA 0.16 times the dry weight of cellulose fibres (CF). The two samples were placed in a sonication bath (USR 170H, Merck Eurolab N.V. with a 215 Watts power). One sample was treated for 30 min and the other sample was treated for 60 min (i.e. the samples were subjected to a sonication energy of 967.5 J/ml and 1935 J/ml respectively). The samples were called CF(0.16)LA-30 min and CF(0.16)LA-60 min respectively.

Reference sample of non-modified cellulose

A reference sample of non-modified cellulose fibres in a slurry was prepared by taking 400 ml from the initial 1.5 wt% disintegrated pulp slurry (i.e. softwood pulp disintegrated according to ISO-5263-1 as described above.

Characterization

Drainage time

Both samples of lactic acid modified cellulose fibres (CF(0.16)LA-30 min, CF(0.16)LA-60 min) and the reference sample of non-modified cellulose fibres were diluted to a suspension or slurry with a dry content of 0.3 wt% (i.e. 1.6 L of water was added to each of the two 400 ml samples). The drainage setup was composed of a glass filter holder for membrane filtration with an inner diameter of 7.2 cm and it was fitted with a Durapore^® filter membrane with a pore size of 0.22 pm. The drainage device was connected to a vacuum of 700 mbar. The drainage time was measured in triplicates for each sample and each sample had a volume of 400 ml. The end point of drainage was assumed to be the instant when the difference between two consecutive drops was more than 30 seconds. All measurements were run at room temperature. The filtration time of 400 ml water (no cellulose fibre present) was also recorded as a reference. The drainage times are presented in Table 6

Results

Table 6

Claims

1. A method for preparation of modified cellulose, the method being characterized in that a mixture of an organic acid and cellulose material in a suspension medium is sonicated until the sonication energy is between 5-1000 J/ml.

2. The method according to claim 1 wherein the concentration of the cellulose material in suspension is from 0.1 to 20.0 wt% before the organic acid is added to the suspension.

3. A method according to claim 1 or 2, wherein the cellulose material is a cellulose nanomaterial and the modified cellulose is a modified cellulose nanomaterial.

4. The method according to claim 3, wherein the cellulose material is a cellulose nanomaterial, such as microfibrillated cellulose (MFC), cellulose nanofibers (CNF), cellulose microfibrils (CMF), nanofibrillar cellulose (NFC).

5. The method according to claim 3 or 4, wherein the concentration of cellulose nanomaterial in suspension is from 0.1 to 5.0 wt%, before the organic acid is added to the suspension.

6. The method according to any one of claims 1-5, wherein the suspension medium is an organic solvent; an aqueous solvent; an aqueous solution; water; or different combinations thereof.

7. The method according to any one of claims 1-6, wherein sonication is performed until the sonication energy is between 5-600 J/ml.

8. The method according to any one of claims 1-7, wherein sonication is performed until the sonication energy is between 100-600 J/ml.

9. The method according to any one of claims 1-8, wherein the ratio of organic acid to cellulose material in the mixture of an organic acid and cellulose material in a suspension medium may be from 0.01:1 to 20:1, as calculated by dry weight of cellulose.

10. The method according to any one of claims 1-9, wherein the organic acid has a pKa of 1.0-6.0.

11. The method according to any one of claims 1-10, wherein the organic acid is selected from any acid of the formula CH₃-(CH₂)_n-COOH,

CH₃-(CH₂)_n-C(OH)-COOH, HOOC- (CH₂)_n-C(OH)-COOH, HOOC-COOH,

HOOC-(CH₂)n-C(OH)(COOH)-(CH₂)_n-COOH,

HOOC-(CH₂)n-[C(OH)]n-(CH₂)n-COOH and Ph-COOH or a combination thereof, wherein each n independently is an integer from 0-6.

12. The method according to any one of claims 1-11, wherein the organic acid is selected from any one of acetic acid, propionic acid, butyric acid, lactic acid, malic acid, citric acid, tartaric acid and benzoic acid.

13. The method according to any one of claims 1-12, wherein the organic acid is lactic acid.

14. The method according to any one of claims 1-13, performed without the presence of a catalyst.

15. The method according to any one of claims 1-14, further comprising a step of removing the suspension medium from the sonicated mixture.

16. The method according to claim 15, wherein the suspension medium is removed by vacuum filtration, pressure filtration or evaporation or a combination thereof.

17. The method according to any one of claims 15-16, further comprising a step where the sonicated mixture is kept at a temperature not higher than 90 °C before removing the suspension medium.

18. The method according to any one of claims 15-17, wherein the modified cellulose material is retained for a period from 0.5 s- 24 hours before removing the suspension medium.

19. The method according to any one of claims 1-18, further comprising a step of drying the modified cellulose nanomaterial to a dry content of at least 98 wt%.

20. The method according to claim 19, where drying is performed by compression moulding or casting the modified cellulose nanomaterial to obtain a film or nanopaper.

21. The method according to any one of claims 3-20, wherein the modified cellulose nanomaterial has a water retention value of from 20-40, as defined by the ratio mass of water to mass of dry cellulose nanomaterial in the modified cellulose nanomaterial after centrifugation of the cellulose nanomaterial, removal of the supernatant and drying the modified cellulose nanomaterial at 100 °C for 24 hours.

22. Use of the method according to any one of claims 1-21, for reducing the time required for water drainage from suspensions based on a cellulose material .